I can't tell you exactly but it is not much, especially compared to a streaming video site. So if you broadband provider is offering 3 plans, for example....
1.5 Mbps
3.0 Mbps
6.0 Mbps
The 1.5 plan is more than sufficient for eBay. Most broadband companies are more than happy to upgrade your plan if you did not purchase enough bandwidth, it is the downgrading if you purchase too much that they are hesitant to do.